Bacteria: Structure , Growth, Taxonomy Flashcards
Explain what bacteria are and a few features
- Unicellular free living microorganisms (in soil, water, hot springs)
- Commensals- one organism derives food or benefits from another microorganism without hurting it
- Many are host-dependent
What kingdom were bacteria originally placed in and how has this changed?
- Placed in Protista (meaning single- celled organisms) to be distinguished from animals and plants
- Yet different to single- celled eukaryotes so placed into kingdom monera (meaning single/ solitary) and protists in separate kingdom
- This changed to prokaryotes (unpaired chromosome and no nucleus) and eukaryotes (paired chromosome and nuclear structure)
- Prokaryotes split into archaebacteria and eubacteria (differences in rRNA)
- Eukaryotes split into protozoa and algae/ slime mould
Which bacteria are medically relevant?
Eubacteria
Outline features of typical bacterial cell
Genome- single circular DNA molecules
- Well organised/ structured organelle- associated with proteins, circular and high level of packing despite no nuclear membrane (NOT just floating around in cytoplasm)
Ribosomes- site of protein synthesis
- different sedimentation coefficient (terminal velocity) and smaller than eukaryotes
Cell membrane- Lipid bilayer- hydrophobic
- Proteins for uptake and secretion of small diffusible molecules
Cell wall- mycoplasmas are the exception- only bacteria that don’t have a cell wall
Outer membrane- another hydrophobic lipid bilayer- differentiates gram +ve and gram -ve bacteria (except for subtle differences in cell wall)
Appendages- e,g, pili from pilling protein- important in adhesion to epithelial cells
- Gonococcus- adhere using pili to urogenital tract
E.g.2 Flagella- move to more advantageous environment for nutrients- anchored to the cell membrane as need energy (ATP) to be motile (move) from ETC
Compare following features of prokaryote VS eukaryote protist Nucleic acids Structured nucleus Mitochondria Ribosomes Cell membrane Rigid cell wall Mesosomes- what is this? ER/gogli
Nucleic acids- P, E Structured nucleus- E Mitochondria- E Prokaryotes get energy from ETC Ribosomes- P, E 80 VS 70 S a Cell membrane- P, E Rigid cell wall- P Mesosomes- P (artefact or invagination of inner membrane important for cell division- only visible under EM- fixing and dehydration could lead to artefact) ER/gogli- E
Outline taxanomic ranks
Kingdom,
Phylum/ Division (same body plan e.g. backbone),
Class (same general traits e.g. no. legs),
Order (additional info e.g. herbivores or carnivores),
Family (closely related genera),
Genus (closely related species),
Species (some variation, essentially same)
Remember: King Prawn Curry Or Fat Greasy Sausages?
No of taxonomic units increase from Kingdom–> species - but no. of organisms occupying taxonomic units decrease until we get to species level where 1 organism occupying this entity
Medical microbiology: Can also have subspecies/ isolates- same bacteria but may have different genes e.g. 1 producing toxin another not
Bacterial naming/ nomenclature- how does this work
Genus + species
Why is bacterial taxonomy important (5)
1) handling information- facilitates storage and retrieval of info through databases
2) Learning- facilitates learning and understanding of large amounts of complex info on diverse org
3) Commmunication- communication about bacteria more accurate
4) Identification- identification of unknown bacteria possible
5) Evolution- provides possible guide to evolution of bacteria allowing epidemiological data to be understood (e.g. in outbreak- where start/ come form)
Classification of bacteria by Phenotypic characteristics- what needs to be considered
- Morphology- macroscopic- on agar plate what do individual cells look like?
microscopic- under microscope? - Biotyping- biochemical tests e.g. sugar fermentation (different bacteria ferment different sugars) and indole test (characteristic of E . coli as indole produced from deamination of aa tryptophan to tryptophanase and kovacs reagent goes bright red in presence of indole)
- Serotyping- differences in antigenic determinants on outer cell membrane- e.g, different types of salmonella
- Antibiogram properties- Different resistances and susceptibilities to different antibiotics
- Pyocin/phage typing:
- Pyocins are bacteriocins- antibiotic proteins produced by bacteria that have activity against other bacteria
- Bacteriophages- viruses that bind to bacterial cells and take over their cell metabolism
Different resistances to these pyocins and phages useful in typing different bacteria
Classification of bacteria by genotypic characteristics- what needs to be considered?
- G+C ratios: Different organisms have different ratios of G+C/A+T
- Campylobacter- food poisoning causes gastroenteritis (inflammation of stomach and intestine)- low G+C ratio (30%)
- Pseudomonas aeruginosa - high G+C/A+T (60%)
- cant use this as an identifying factor past genus level
- DNA hybridisation: Isolating genome, breaking 2 strands and probing with single-stranded small probes specific to certain species of bacteria-
- if probe binds indicative of type of bacteria dealing with
- Nucleic acid sequence analysis- e.g. MLST- multi local sequence typing- used to different subspecies of bacterium
- Looking at housekeeping genes- needed for growth and sustainability of organism
- Mutations within these genes within isolates (subspecies!) of the same species
- Use type mutations to put them in different regions of database
- Chromosomal DNA fragment Analysis: Extracting DNA, enzymically breaking this (cutting with restriction enzymes) and running it on agarose gel (polymer) with electrophoresis- checking for differences in banding pattern
- Ribotyping: looking at ribosomal RNA- within these genes are hypervariable regions which are different
- Differences within these can identify bacteria across different species and genus divides
- Clostridium difficile (infection of large intestine- colon- rare) has very good ribotyping system to recognise different isolates
Within macroscopic morphology what is it important to look for?
- Texture (smooth or rough)
- Appearance- glistening or dull
- Pigmentation- non-pigmented (white, cream, tan, colourless) or pigmented (purple, red, yellow)
- Optical density (opaque/ translucent- indicative against dark background)
Explain appearances of some colony morphology
- E. coli- translucent whereas Staph. aureus opaque when grown on agar
- On blood agar can have a-haemolysis (incomplete breakdown of RBC- hence green colour) or b-haemolysis (complete breakdown of RBC)
e. g. Str. pnuemoniae (a-haemolysis) and resistant to optochin (antibiotic) VS Str. pyogenes (b haemolysis) and resistant to bacitracin
Explain microscopic morphology (morphology of single cells)
Shape
- Rod/bacilli
- Club (thicker rod)
- Coccus (round)
- Curved (vibrio)
- Spirilllum
- Spirochaete
Arrangement
- Individual
- Grouped
Size
- difficult to differentiate (microbiologist)
- Staining characteristics
- Gram stain- identify true bacteria
- Acid fast stain- mycobacteria have different cell wall so cant use gram stain, use acid fast stain
What is gram staining used for
- First line of diagnosis- shows shape and size and good for empirical antibiotic treatment
- Differentiates based on cell wall
- Gram +ve= thick cell wall
- Gram -ve= thin cell wall
Explain steps in gram staining
1) Fix sample to slide so not removed when adding all solutions
2) Add crystal violet- dye which stains all bacteria purple colour
3) Gram’s iodine- acts as mordant- fixes dye into cell wall
4) Add decolouriser- type of alcohol or acetone- removes crystal violet from thin gram-ve wall but cant be removed from thicker gram +ve cell wall so remain purple
5) Add counter stain- so gram -ve take up red safranin red stain
As gram +ve not lost original crystal violet purple stain- retain this colour
